In coherent optical communication, quadrature modulation is employed in which amplitude modulation is independently performed for each of an in-phase component (I component) and a quadrature phase component (Q component). The increase in transmission rate has been achieved by multi-level modulation such as QPSK (Quadrature Phase Shift Keying) and 16QAM (Quadrature Amplitude Modulation). For a further speed-up, level multiplication to 64QAM or the like has been also promoted. On the receiving side, an optical signal is converted into an electric signal by an optical demodulator, and after A/D conversion, the distortion of a transmission path is compensated. Therefore, by digital signal processing, chromatic dispersion compensation, polarization processing/adaptive equalization and error correction are performed, leading to an increase in receiving sensitivity.
As a problem that becomes conspicuous in the case of using the multi-level modulation such as QPSK, 16QAM and 64QAM, there is constellation distortion (IQ distortion). A multi-level modulated signal is treated as an electric signal with four lanes (the I component and Q component of an X polarized wave and the I component and Q component of a Y polarized wave), at an electric stage. That is, on the transmitting side, the signal is generated as an electric signal with four lanes, and is converted into a multi-level modulated signal by an optical modulator.
As the optical modulator, for example, a Mach-Zhebnder interferometer type modulator is applied. Such an optical modulator has imperfection due to errors of bias voltage, a finite extinction ratio of the interferometer and the like, and by such an imperfection, constellation distortion is generated. When constellation distortion is generated, the sent information cannot be exactly decoded, causing an increase in bit error rate, and the like. Here, a constellation is also called a signal space diagram, and a data signal point by digital modulation that is shown on a two-dimensional complex plane (a point that is shown by the I component and Q component of the complex plane).
For example, the 16QAM and the 64QAM are modulation schemes having constellations with 16 points and 64 points respectively, and generally, the 16 points and the 64 points are arranged on a signal space in square shapes respectively. The 16QAM can be regarded as a modulation in which four-level amplitude modulations independent from each other are performed to the in-phase component and quadrature component respectively, and the 64QAM can be regarded as a modulation in which eight-level amplitude modulations independent from each other are performed to the in-phase component and quadrature component respectively.
As one kind of constellation distortion, there is a DC (Direct Current) offset. Typically, a bias voltage is applied to the optical modulator, such that the optical output is a null point. When the bias voltage shifts from the null point, the DC offset is generated. Further, in the Mach-Zehnder interferometer constituting the optical modulator, it is ideal that the optical output is absolutely zero when the extinction ratio (ON/OFF ratio) is infinite, that is, OFF. However, when the optical output is not absolutely zero at the time of OFF, the extinction ratio is not infinite, and the DC offset is generated. The DC offset appears as a remaining carrier in the optical signal, and therefore, can be confirmed by observing the spectrum of the optical signal.
The DC offset and the remaining of the carrier due to this are caused also by a direct detection scheme that is not a coherent detection scheme using a local oscillating laser (for example, a scheme of a directly detecting the intensity of an ON-OFF signal of 1010 with a photodetector, which is also called an intensity modulation direct detection and the like). In the direct detection scheme, the remaining carrier appears as the DC offset again, at an electric stage on the receiving side, and therefore, can be easily removed by an analog DC block circuit having a capacitor and the like. On the other hand, in the coherent detection scheme, when there is no exact coincidence in frequency between a transmitting laser and the local oscillating laser on the receiving side, the remaining carrier is not converted into direct current at the electric stage on the receiving side, and cannot be removed by the DC block circuit.
Further, as constellation distortion, IQ (In-phase Quadrature) crosstalk is known. The IQ crosstalk occurs when the phase difference between the in-phase component and the quadrature component is not exactly 90° due to a bias voltage error of the optical modulator.
For coping with these problems with constellation distortion, there is disclosed a technology of previously measuring the characteristic of optical modulator to be applied in an optical transmitting device and compensating the characteristic of the optical modulator with a digital signal processing device in the transmitting device (for example, see NPL 1). Further, there is disclosed a technology of calibrating, on the receiver side, a distortion called a quadrature error that is caused by the gain unbalance and phase unbalance between the I-Q signal components, when the quadrature modulation is used in wireless communication (for example, see PTL 1).